This is a study of mate selection occurring at the population boundaries of an American Indian population that is highly mobile and has sharply increasing admixture rates. It is an attempt to determine (1) what biological, residential, and socio-cultural factors are leading to out-mating (either directly or indirectly by affecting travel and mobility), and (2) to relate these processes to the genetic and morphological structures, reproductive levels, and the distribution and restructuring of the population. Stratified samples of mated Papago couples will be drawn from reservation and off-reservation sites in Arizona and California. Abbreviated individual histories will be constructed focusing on activities, visits, and residences at various ages to determine relative to the time of marriage(s) what factors were important in conceiving each mating. Reproductive levels and the genetic and morphological structures of each mating type will be measured. Information on the parents, sibs, and offspring of each mating in the sample will be included to trace the effect of out-mating on population distribution. Selective out-mating will be investigated for skin color through reflectometry studies and 10 anthropometric measurements reflecting body size and shape will be taken. Homogeneous matings will serve as controls for heterogeneous matings, reservation sites as controls for data sampled off-reservation.